The present invention relates to the video camera art. It finds particular application in conjunction with quality control and monitoring with video cameras, especially of continuous material processes, and will be described with particular reference thereto. It is to be appreciated that the invention may find other applications including document reading, photographic archival recording, object tracking, video security, and the like.
Heretofore, quality control and monitoring has been carried out with charge coupled devices (CCD) and other video cameras. In one method, a video output signal was generated which included a long, continuous series of video image fields. In a frame transfer CCD camera, light from a continuous or pulsed source was focused on an image section of a CCD sensor for a selected interval of time. The interval was selected to produce good image contrast without significant blurring of the image due to object motion. The charge on each element of the image section was indicative of received light intensity. The charge was transferred during a vertical blanking interval, e.g. a few hundred microseconds, into corresponding elements of an optically insensitive CCD mass storage section. As the image section again commenced integrating received light, the charge was read out element by element from the optically insensitive elements to form a video signal representing one field of the resultant image. After the 1/60th of a second or other selected read out interval, the charge representing the second field was transferred from the image section to the storage section. As the second field was read out of the storage section, the second video signal image section started integrating light to form a third field. This sequence was repeated cyclically to form a video signal representing a series of single image fields.
Continuous production of image fields rendered CCD cameras awkward to adapt for certain high volume quality control situations. As a continuous sheet or individual object was moved past the CCD camera, the resultant video signal represented a long series of image fields. In order to review the images of each object to monitor for a controlled characteristic, it was first necessary to determine which portion of the video signal included the field(s) which represented the monitored individual object or portion of the continuous sheet. Second, it was necessary to determine within the field the actual location of the monitored object or sheet portion. When increased lighting was necessary, the actuation of a strobe light was coordinated with the field of interest. If the strobe light was not completely coincident with a common location of the object or sheet portion within the field(s) of interest, lighting intensities and object shapes would vary among the fields of interest for each object or sheet portion. If the stream of objects or sheet was moving rapidly compared with 1/60th of a second or other one field exposure time, then each object would be in a different position within the selected field of interest. This different positioning of the object not only required identifying the object position in the video field, but could also result in different lighting conditions on the object. These inaccuracies in the timing, positioning, and lighting of the monitored objects all limited the degree of accuracy and the speed with which quality control monitoring could be performed.
In the quality control and monitoring method described in parent patent application Ser. No. 186,446, filed Apr. 28, 1988, a CCD device is asynchronously triggered at a controlled instant in time to "grab" a moving object. The instant in time is synchronized with the moving object's entry to a preselected examination point. A high intensity strobe is flashed concurrently with asynchronously triggering a CCD device to "grab" the moving object. While such a method has certain unique advantages, it requires a significant amount of power capacity to flash the high intensity light necessary for its functioning. The minimum cycle time of the strobe limited the speed of the conveying system.
Although asynchronous triggering is applicable to continuous web monitoring, some webs are advanced at such high speeds that the repower time of the strobe may limit the web advancement speed. Additionally, inspection of continuous webs with cameras producing a series of individual fields that requires matching the tops and bottoms of adjacent fields to provide a single, complete image of the web without gaps or overlaps. Processes in which continuous webs are advanced include the fabrication of sheets and films of plastics such as polyethylene, MYLAR, cellophane and vinyl, metals, glass, plywood, paper and other wood pulp products, fabrics, printing of newspapers, magazines, wallpaper, packaging, etc., lamination of plastics, composites, paper, etc., coating of plastics, metals, etc. with paint, magnetic particles, abrasives, adhesives, photographic emulsions, electrically conductive materials, etc., and embossing, cutting, slitting, perforating, etc. of any of the aforementioned raw or processed materials.
Previous inspection of continuous web materials was carried out using either CCD cameras in the raster-scan mode with stroboscopic illumination or by using line-scan cameras with intense continuous illumination. Line scan cameras were constructed with a single row of photosensitive areas or sensors. A large amount of illumination was necessary to produce usable signals from the sensor. Because the sampling of the single line of sensors was controlled by an external clock, there were gaps or overlap in the monitored web, depending on the speed of the web.
Both the raster-scan and the line scan cameras created distortions in the image data. In the case of the raster-cameras, the top and bottom edges of the images had to be found and the overlap or missed material corrected. In the case of line-scan cameras, the increment of material covered by each scan could be different if web speed changed.
Even if the line-scan and raster-scan cameras were synchronized with the moving web, problems still remained. In the case of the raster-camera, the top and bottom edges still had to be matched and high power, stroboscopic illumination was required. In the case of line-scan cameras, the time that the line of photodiodes was exposed to light was very short. Accordingly, very brilliant illumination was required. The light requirements, whether stroboscopic or high intensity, became so burdensome that the maximum speed of the web material was limited.
The present invention contemplates a new and improved video camera system and method which overcomes the above referenced problems and others.